1. Field of the Invention
This invention relates to a lead-free multi-layer sliding part and to a method for its manufacture. Examples of a sliding part according to the present invention are a cylindrical sliding member such as a bushing for use in a radial sliding bearing (also called a journal bearing) and a planar sliding member for use as a swash plate in a compressor, pump, or hydraulic motor.
2. Description of the Related Art
Sliding bearings are often made of a multi-layer sliding material comprising a backing plate of steel and a layer of a bearing metal bonded to the backing plate in order to increase the strength of the bearings so that they can withstand high loads.
For example, a bushing, which is a cylindrical sliding member used in a radial sliding bearing, is typically made of such a multi-layer sliding material. Such a bushing can withstand higher loads than a ball bearing or a roller bearing. Therefore, radial bearings using such a bushing are frequently used in portions on which a very high load is exerted, such as on the undercarriage of construction equipment such as bulldozers or power shovels to support rollers, or in the drive portions of automobiles in which a high load is applied and high speed rotation takes place.
A multi-layer sliding material is also used to form a planar sliding member which is subjected to high loads, such as a swash plate of a compressor or hydraulic motor, for example.
Methods for joining a bearing metal layer to a steel backing plate include the cladding method, the casting method, and the powder metallurgy method.
In the cladding method, the bonding strength between the bearing metal and the steel backing plate is relatively low, so when a high load is applied to a sliding bearing made from such a multi-layer material, there are cases in which the bearing metal layer peels off the steel backing plate.
In the casting method, a molten bearing metal is cast atop a steel backing plate. Although a bearing metal layer formed by the casting method has a strong bonding strength to the steel backing plate, this method is very troublesome to perform, and at the time of casting of the molten bearing metal, oxides can be included or shrinkage cavities can form, and these may have an adverse effect on bearing properties.
In the powder metallurgy method, a bearing metal in the form of a powder is dispersed on a steel backing plate and is sintered to join the grains of powder to each other and to the backing plate. A multi-layer material which is obtained by the powder metallurgy method has a strong bonding strength between the sintered bearing metal layer and the steel backing plate, and the sintering temperature can be made low, so there are few thermal effects on the steel backing plate or the bearing metal such as can occur with the casting method.
The materials which have been most commonly used to form a bearing metal layer of a multi-layer sliding bearing have been Cu-based alloys such as lead bronze, phosphor bronze, and high strength brass.
Lead bronze has good bearing properties, but it is a potential source of environmental pollution. When equipment containing a multi-layer bearing is discarded, the bearing is usually disposed of by burial underground, since the bearing metal layer and the steel backing plate cannot be readily separated from each other and so cannot be easily recycled. If a multi-layer bearing containing lead bronze which has been disposed of in a landfill is contacted by acid rain, lead may be dissolved from the bearing metal layer by the acid rain and may pollute underground water. If underground water which has been polluted by lead in this manner enters the water supply and is drunk for long periods by humans or livestock, the lead accumulates in the body and may cause lead poisoning. For this reason, the disposal of lead-containing materials is being increasingly regulated, and there is a trend in industry away from the use of lead-containing materials such as lead bronze for bearing metal layers of sliding bearings.
Phosphor bronze and high strength brass do not contain lead, but their sliding or bearing properties may not be adequate when they are subjected to high instantaneous loads, such as are applied to a swash plate of a compressor or hydraulic motor at the start of operation. As a result, these materials are more susceptible to seizing.